Because the water when heated, becomes lighter, and stays at the surface, while the cold water is heavier and remains at the bottom of the vessel.
Where must heat be applied to warm a liquid or a gas?
It must be applied to the bottom of the vessel containing the liquid or the gas.
Where must a fire be, so as to warm a room?
The fire must be near the floor of the room.
Why does a hot substance cool quickly in the air?
Because the air touching a substance ascends as soon as heated, and cold air takes its place; as this process goes on rapidly, the substance is quickly cooled.
How is water cooled by convection?
The water at the surface is cooled by giving off its heat to the air; it then falls to the bottom, and the warm water rises to take its place at the surface.
Why does stirring hot coffee cool it quickly?